U.S. Pat. No. 5,705,902 of Merritt et al. discloses a DC motor/generator based on a Halbach array of permanent magnets. This motor/generator is "inside-out" when compared with a conventional motor/generator: its rotor is on the outside of the machine, and its stator is on the inside. An array of permanent magnets rotates along with the rotor.
This geometry creates new mechanical challenges. When the rotor operates, it expands radially. The permanent magnets increase the centrifugal expansion by adding parasitic mass to the inner surface of the rotor. Large degrees of radial expansion threaten to pull the rotor away from other components of the motor/generator, thereby causing mechanical failure.
U.S. Pat. No. 4,860,611 of Flanagan et al. discloses a flexible rim hub for an energy storage rotor. Flanagan's metal hub is designed for use within the rotor itself. A tight interference fit within the rotor inhibits easy removal of the hub during maintenance. Flanagan's metal hub design is not a suitable mechanical attachment outside the axial length of the rotor. The hub lacks a surface through which bolts might be used to attach the hub to an outer axial end of a rotor. Moreover, because outer rim sections of Flanagan's hub between the spokes would expand more than rim sections at the end of spokes, the hub would be prone to failure if not surrounded by the rotor. Therefore, such a hub cannot be mounted outside the axial length of the rotor because, with its rim unenclosed by the rotor, it expands unevenly around its circumference and could break apart.
There is a need for hubs that reduce mechanical strain in motor/generators, while also being easily accessible for maintenance.